The field of the invention relates to exhaust aftertreatment devices for internal combustion engines. In particular, the field of the invention relates to selective catalytic reduction (SCR) systems for internal combustion engine exhaust including those suitable for use in industrial processes and in mobile and stationary diesel, natural gas, and other engine applications.
Exhaust from internal combustions engines typically includes oxidized nitrogen gases such as nitrogen oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), which collectively are referred to as “NOx.” Because NOx can be an environmental hazard, systems have been developed to remove NOx from exhaust by selective catalytic reduction (SCR).
Selective catalytic reduction (SCR) of nitrogen oxides (NOx) using a reducing agent is known in industrial processes as well as in stationary diesel engine applications. Ammonia is a commonly used reducing agent in SCR systems. NOx reacts with ammonia and is catalytically reduced by a SCR to nitrogen gas (N2) with water as a by-product. This reaction may be represented generally as follows:NOx+NH3→N2+H2OThis reaction may be catalyzed by catalysts referred to as “SCR catalysts.”
Because ammonia is a hazardous substance, typically ammonia is not directly introduced into SCR systems. Rather, ammonia is generated in situ by introducing a less hazardous, ammonia-generating source into the SCR system. One common ammonia-generating source is aqueous urea. In the SCR system, aqueous urea is injected into the exhaust gas flow stream upstream of the SCR. Water in the droplets from the injection solution evaporate leaving urea, which decomposes in heat (i.e., pyrolyzes) to isocyanic acid and ammonia. In water, isocyanic acid then hydrolyzes to create ammonia and carbon dioxide. These reactions may be represented generally as follows:CO(NH2)2+heat→HNCO+NH3 HNCO+H2O→NH3+CO2 These reactions may be catalyzed by catalysts referred to as “pyrolysis catalysts” and “hydrolysis catalysts,” respectively. The ammonia thus formed may react and reduce NOx in a SCR reaction.
The use of SCR catalysts for mobile applications is problematic. One obstacle is that mobile applications are relatively small in size. This makes it difficult to inject a urea solution into exhaust upstream of an SCR catalyst and decompose and hydrolyze the urea solution completely to ammonia prior to the exhaust arriving at the SCR catalyst (and achieve a high enough ratio of ammonia to NOx). At low diesel engine exhaust temperatures and high gas flow velocities (e.g., about 20 meters per second), a distance of several meters (i.e., a time of 0.1 to 0.3 seconds) between the injector and the SCR catalyst is required for the aqueous urea solution spray to vaporize, for urea to decompose and hydrolyze into ammonia and carbon dioxide, and for the ammonia to become practical uniformly distributed across the flow front before entering the catalyst. Although various solutions to this problem have been suggested, (see, e.g., U.S. Pat. Nos. 6,928,807; 6,361,754; and 6,203,770; and U.S. published application No. 2006/0115402), these solutions involve generating ammonia outside of the SCR system and introducing the ammonia directly into the exhaust stream. In these systems, if the SCR catalyst is not pre-heated prior to introduction of this ammonia, the ammonia will pass through the SCR catalyst and foul the air. Therefore, there is a need for improved SCR systems that are suitable for mobile applications.